In order to efficiently generate electricity using a polymer electrolyte fuel cell for example, it is important that a plurality of individual cells stacked on one another each contain moisture in an appropriate amount that is not too small or not too large. To this end, conventionally, a dry state or an overly humid state of a cell has been detected from a cell voltage or the like, and the humidity of the cell has been controlled on the basis of the detected state, so as to be recovered to an appropriate humidity level (Patent Documents 1 and 2). Patent Documents 3 through 6 disclose related techniques.